1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having a plurality of circuit elements including a vertical MOSFET isolated from the other circuit elements.
2. Description of the Related Art
Recently, vertical MOSFET's have been employed as switching devices for driving loads, such as lamps and solenoid relays. There has recently been proposed an integrated circuit device wherein a vertical MOSFET and a plurality of peripheral circuit elements are integrated on the same one chip by making use of the advantages that the process for manufacturing a vertical MOSFET and a CMOS IC on the same chip is developed and that various kinds of protecting circuits such as a current-limiting circuit, an overheating-detection circuit, an overvoltage-detection circuit are put into practice by using these peripheral circuit elements to protect the vertical MOSFET from a large current due to a short-circuitting of its load or an application of a high-voltage surge.
In the integrated circuit device having a vertical MOSFET as an output transistor and other circuit elements formed on the same chip, the vertical MOSFET must be isolated from the other circuit elements. A junction isolation, and a dielectric isolation are examples of the isolation structure in the prior art.
FIG. 5 shows the junction isolation structure in a prior art (see IEEE 1987 CUSTOM INTEGRATED CIRCUIT CONFERENCE, p. 276), while FIG. 6 shows the dielectric isolation structure in another prior art (see Japanese Patent Laid-Open No. 196576/1986). These conventional isolation structures suffer, however, from the disadvantage that the manufacturing process is complicated and the production cost is high.
For example, in the junction isolation structure shown in FIG. 5, after an N.sup.+ buried layer 51 is provided in an N.sup.+ -substrate 1, a P-type epitaxial layer 52 and an N-type epitaxial layer 3 are stacked thereon successively. Then, a P-type impurity is diffused into the N-type epitaxial layer 3 from the surface to form a P-type diffused layer 53 for isolation. Thus, this structure requires a complicated process.
In the dielectric isolation structure shown in FIG. 6, after the bottom surface of an N.sup.+ -substrate 63 is oxidized to form an internal oxide film 62 for isolation, the internal oxide film 62 in a region where a vertical MOSFET 23 is to be formed is partially etched away. Then, an N.sup.+ polysilicon layer 61 is deposited on the bottom surface of the N.sup.+ -substrate 63, followed by growing an N.sup.- epitaxial layer 3 on the upper surface of the N.sup.+ -substrate 63. Finally a trench 64 is provided to effect isolation by filling phosphosilicate glass (PSG) 11. This structure requires to effect alignment between the obverse and reverse sides of the substrate 63 and to provide the trench 64 having a relatively deep depth. Thus, the manufacturing process includes technically difficult steps.
As shown in FIGS. 5 anc 6, vertical MOSFET uses the N.sup.+ -substrate 1 or 63 as its drain region. Therefore, when the drain region is directly connected to an output terminal, a load is connected between the output terminal and the positive or negative power source line. The voltage at the drain region of vertical MOSFET changes in accordance with the output state. On the other hand, the potential at the substrate 3, 63 of the other CMOS circuit portion 26 and the potential at a well 4 need to be fixed. Therefore, the substrate and the well for other circuit portion 26 must be isolated from the drain region of the vertical MOSFET. Accordingly, it is necessary to electrically isolate the vertical MOSFET from the other circuit elements by use of an isolation structure such as the above-described junction isolation or dielectric isolation.
On the other hand, in automotive electrical circuits, the automobile body itself is used as a grounding electrode with a view to reducing the number of interconnections. In the case where the loads such as lamps and solenoid relays in motorcars are driven by using the vertical MOSFET, the loads are connected with the automobile body for preventing them from breaking down by a surge voltage introduced into the positive power line by sparking in engine. This results in an connection of the vertical MOSFET operating as switching devices for driving these loads between those load and the positive power source line. This type of switching device is known as a high-side switch.
The high-side switch may be formed by using an N-channel vertical MOSFET. The drain of the N-channel MOSFET is connected to the positive power supply side and the source thereof is employed to constitute an output terminal which is connected to a electrode of a power load such as a lamp and a solenoid relay for motorcars.
As described above, in the high-side switch, the output terminal is connected to the source electrode and the potential at the drain electrode is fixed to a power supply voltage which is common to the other circuit elements. Therefore, it is possible to form a vertical MOSFET and the other circuit elements on a common substrate. However, since the vertical MOSFET used as an output transistor is employed to switch a high voltage and a large current, a large current flows between the source of the vertical MOSFET and the drain region thereof which is common to the substrate of the other circuit elements, resulting in changing the substrate potential. If the substrate potential near the other circuit element lowers, the PN junction between the substrate and source or drain region becomes forward bias to cause latch-up in the portion of the other circuit. Accordingly, a current path must be limited so that there is no adverse effect on the other circuit elements by contriving the device structure. In this case, the device structure must also be contrived to reduce the required numbers of manufacturing steps for easiness of manufacture and low production cost.